1. Field of the Invention
The present invention is directed to a process for manufacturing a molded piston ring for a piston of a piston-cylinder unit.
2. Description of the Related Art
German patent publication 44 10 996 C1 discloses a flat-topped piston for a vibration damper the exterior surface of which is covered by a piston ring. According to the description, the piston ring can be made of plastic and injected on or, alternatively, rolled onto the piston. This is achieved using a two-shell or two-sheet strip material comprising a reinforcement and a sliding strip.
A plastic piston ring is disadvantageously burdened by even the slightest possible axial play. To overcome this problem, the piston ring may be pretensioned, as for example taught by German Patent publication 40 32 609 C1.
Another problem associated with manufacturing the piston ring using a plastic material is that since a plastic piston ring is subjected to extensive thermal expansions a space or by-pass results between the inner wall of the cylinder and the piston ring at relatively low temperatures and increased friction in the vibration damper at elevated temperatures.
Plastic piston rings can also be produced as individual parts by injection molding. These individual parts have a joint and are pinned to the piston and thus require considerable production efforts and exhibit significant axial play. In a piston ring with a cross-section shown in FIG. 3 of German Patent publication 44 10 996 C1, undercuts occur as a result of the covered surfaces holding the piston ring at the piston which can be eliminated or controlled only by special molding techniques, especially the releasing of the piston ring from the injection mold, that are significantly more complex and accordingly more expensive.